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14 the Federal Water Power Act for the construction of hydroelectric dams in “navigable waters.” 311 U. S., at 406–410. Only after applying the Daniel Ball definition to determine that the river in question was navigable did the Court hold that Congress had plenary authority over the erection of structures in the river, regardless of whether the structure actually impeded navigability. 311 U. S., at 423–426. While this represented an expansive application of the old concept that Congress can prevent obstructions to navigable capacity, see, , Appalachian Elec. made clear that the term “navigable waters” remained tethered to Congress’ traditional channels-of-commerce authority—not to the broader conceptions of the commerce authority adopted by the Court at that time.

The next year, in Oklahoma ex rel. Phillips v. Guy F. Atkinson Co., 313 U. S. 508 (1941), the Court reaffirmed that the term “navigable waters,” this time as used in the Flood Control Act of 1936, was to be interpreted in light of the expanded Daniel Ball test. 313 U. S., at 522–525. Significantly, Oklahoma was decided mere months after Darby, one of the most significant cases expanding the scope of the commerce authority. 312 U. S., at 119. However, Oklahoma did not so much as mention Darby in construing the jurisdiction Congress conveyed in the term “navigable waters.” Instead, it cited Darby only in passing and to support the argument that, once a river is deemed navigable under the channels-of-commerce authority, Congress has authority to protect “the nation’s arteries of commerce” by regulating intrastate activities on nonnavigable parts and tributaries of the navigable river lest such activities “impai[r] navigation itself.” Oklahoma, 313 U. S., at 525. This was nothing more than an application of the principle that Congress can regulate activities that obstruct navigable capacity. Thus, even as the Court expanded the Commerce Clause in other contexts, it continued to understand that the term “navigable waters” refers solely to the aquatic